I believe we haven't found the whole story yet so I'll post as a partial answer for the moment.
Looks like we may have found the answer! I'll leave this for anyone interested in some extra information or my rambling speculations.
Soyuz MS-09 launched from Baikonur LC-1 Gagarin's Start which is shown below. I've highlighted the flame trench (green), outbuildings (yellow) and what I believe are the pillars (red).
Soyuz typically launches roughly East-North-East from Baikonur and only begins its pitch-over some 20 seconds into the flight; the pillars seem to be some 150-200 metres North of the launchpad and quite a small target, making it seem very unlikely they are for controlling falling debris. However, I found a further (unsubstantiated) comment that corroborates this purpose (Google translated - note 'rocket' and 'missile' seem to translate the same):
In the foreground of the pile. This is one of the security measures for emergency situations during the launch of the missile. They serve as a sort of "dissector". In the event of a crash, a missile (and, or debris), falling on a field of often spaced concrete pillars, collapses, the radius of debris scattering on the surface decreases.
Furthermore, the flame trench is (intentionally) directed away from any buildings and some of the buildings are clearly not protected in any way by the pillars from the flame trench.
As pointed out by OrganicMarble, it could simply be the remains of the foundations of a demolished building. Or it could be the foundations of a structure yet to be built.
Speculation: The pillars also reminded me of the Holocaust Memorial and I considered that this may be something similar. However, I believe the largest loss of life at Baikonur was the Nedelin Catastrophe in which 78 people died. There are far more than 78 pillars (possibly not important) and that incident already has a memorial.
I realise this seems mostly just a rebuttal to the other answers, but I'm just documenting my investigation thus far.
The pillars can be clearly seen in the background of this photo from Soyuz TMA-18M in 2015: